cycles volume rendering 3D image texture (CT or MR dataset)
I've had success importing CT data into blender using the free 32bit version of the open source DICOM viewer OsiriX, free registration required (1).


Import your data into OsiriX
Double click the image-sequence to open it in the 2D Viewer
Choose "3D Surface Rendering" from the "2D/3D" button in the middle of the toolbar
Select the desired level of detail and (most important) the brightness of the pixels which should be considered the "object" (2)
Use the "3D-SR Export" button in the new "Surface Rendering" window to choose your preferred output format (.rib, .vrml, .iv, .obj or .stl)


I went with .obj and it imported smoothly into blender.

(1) OsiriX is Mac OS X only. The site http://www.idoimaging lists free medical image viewing software for many platforms. Maybe there is one with a similar feature set…

(2) Maybe you have to experiment a little in finding the right values. The available presets for Skin, Bones and Metal usually work just fine, but I have to admit, that I have no medical knowledge and therefore cannot say if this would be the case with all the CT/MR data sets out there.